Some info about dithering:
"If temporal dithering is enabled on your graphics card, it may force even static images to be dynamic to emulate a color that is not available in its color palette. Temporal dithering does this by constantly switching between two nearby colors very quickly on pixels on the image. The alternating between the two colors is fast and most people should only see one color, instead of two rapidly changing colors."

I'm using PCoIP that is why I want to disable dithering.
I found how disable it in Windows and now I need the same but for Linux.

AMD GPU dithering is supposed to be happening only when you are using color profiles. In that case, it would use a 10-bit LUT and output an 8-bit signal with dithering enabled to eliminate gradient banding. Otherwise, there should be no dithering if you do not have a color profile loaded.

If you're using a color profile, you could stop dithering from being used by truncating color profile gamma curve output values from ~16-bit to 8-bit. I have code for this so if you're using a color profile, send it to me via PM and I'll convert it to 8-bit.

"...only when you are using color profiles." - I've only installed 'AMD Catalyst™ 13.12 Proprietary Linux x86 Display Driver'. I did not configure it. Is it using some color profile by default? How can check it? How can I disable using color profiles if it'll use?

Open Catalyst Control Center
Go to information|software and look for the 2D driver file path which will have the registry location you're after.
Inside the registry location, create a DWORD value called DP_DisableDither and set it to 1

- the tool itself might have been renamed to "amdconfig"
- the parameter may have been renamed to --set-pcs-u32
- any of those lines require "MCIL," at the beginning
- this only works for the proprietary "fglrx" driver from AMD; it does not work with the open source "radeon" driver which most distributions load by default (not sure if it dithers)
- the changes should be written to /etc/ati/amdpcsdb